Method for the reduction of nitrogen oxides

ABSTRACT

A process for the catalytic reduction of NO x , the reduction taking place in the presence of a catalyst which comprises 
     (a) from 20 to 97 wt % of A 2  O 3 , 
     (b) from 1 to 40 wt % of CuO, 
     (c) from 1 to 50 wt % of ZnO, 
     (d) from 1 to 40 wt % of Ag, 
     (e) from 0 to 2 wt % of Pt, 
     (f) from 0 to 20 wt % of oxides of rare earth metals, elements of the 3rd subgroup of the Periodic Table of the Elements or mixtures thereof, 
     based on the total weight of the components (a) to (e), which adds up to 100 wt %, wherein, in each case, up to half the weight of the component (a) may be replaced by Fe 2  O 3 , Cr 2  O 3 , Ga 2  O 3  or mixtures thereof, of the component (b) by CoO, of the component (c) by MgO, of the component (d) by Au and of the component (e) by Pd, Ru, Os, Ir, Rh, Re or mixtures thereof, is used for reducing NO x , especially in combustion off-gases, the components (a), (b) and (c) forming a spinel which is doped with the components (d), (e) and (f).

The invention relates to the use of certain catalysts for reducingnitrogen oxides (NO_(x)) and to a corresponding process. Nitrogen oxidesprimarily come from combustion off-gases, in particular frominternal-combustion engines such as diesel engines.

The combustion of hydrocarbons with air as an oxidant gives rise,particularly with excess air and at high temperatures, to nitrogenoxides via oxidation of the nitrogen present in the air. Examples ofsuch nitrogen oxides are NO, NO₂, NO₃, N₂ O₃, N₂ O₄ and N₂ O₅. Beingpollutants, the nitrogen oxides are to be removed as completely aspossible from the combustion off-gases, to avoid environmentalpollution. While emissions from power stations and industry areprogressively declining, owing to the use of off-gas treatment plants,abating the pollutant fraction in motor vehicle exhaust gases isbecoming increasingly important, particularly given the increase in thenumber of motor vehicles.

Many solutions have been proposed for abating NO_(x) emissions of motorvehicle engines. Effective solutions for abating the amounts of NO_(x)must meet numerous criteria, particularly if catalysts are used, forexample:

High conversion ratio, ie. extensive removal of NO_(x), even at high andlow temperature and in the event of frequent load changes duringoperation

Avoiding the use of auxiliary materials such as ammonia or urea

Low production and operating costs

Long on-stream time

Low N₂ O production

High mechanical catalyst stability

A number of catalysts for reducing nitrogen oxides have been proposed.

EP-A1-0 687 499 describes spinel catalysts made of a spinel comprisingcopper, zinc and aluminum, for reducing nitrogen oxides.

U.S. Pat. No. 3,974,255 describes a magnesium aluminate spinel catalystcoated with platinum. The catalyst is employed for reducing NO_(x) inexhaust gases from internal-combustion engines.

EP-B1-0 494 388 describes a process for removing nitrogen oxides from anoff-gas in which the catalyst used is a polyvalent metal phosphate, apolyvalent metal sulfate or a spinel aluminate of a transition metal ofthe 4th period of the Periodic Table of the Elements. In particular, acobalt aluminate catalyst is described which is prepared bycoprecipitation of cobalt nitrate and aluminum nitrate, followed bydrying and calcination.

JP-A2-08024648 describes catalysts for removing nitrogen oxides from anoff-gas. The catalyst, for example, has the composition Ag₀.01 P₀.01Pr₀.01 Cu₀.2 Zn₀.5 Al₂.0, and Fe₀.02 Co₀.02 or 0.01% of MgO may also bepresent. The catalyst is prepared by the nitrates of Ag and Pr beingmixed together with H₃ PO₄ and this mixture being introduced into oxidemixtures of Cu, Zn and Al in the presence of aqueous solutions ofammonia or ammonium carbonate or ammonium sulfate. The calcination iscarried out at 500° C. and 800° C., respectively. The silver content ofthe catalysts is 0.67 wt %.

Also known are catalysts for the catalytic decomposition of dinitrogenmonoxide (N₂ O). DE-A1-42 24 881 describes a silver-containing aluminumoxide supported catalyst and a process for the catalytic decompositionof dinitrogen monoxide, either pure or present in gas mixtures, wheredinitrogen monoxide is decomposed selectively without other nitrogenoxides being decomposed to a significant extent into the elements. Thecatalyst used may be in the form of copper/zinc/aluminum spinels whichare doped with silver. This involves, for example, an aluminum oxidesupport being impregnated with a solution of copper nitrate and zincnitrate, being dried, calcined and then being impregnated with asolution of silver nitrate, being dried and calcined.

It is an object of the present invention to provide improved catalysts,compared with known catalysts, for reducing NO_(x). In addition, aprocess for reducing NO_(x), particularly in exhaust gases frominternal-combustion engines, is to be provided.

We have found that this object is achieved, according to the invention,by the use of a catalyst which comprises

(a) from 20 to 97 wt % of Al₂ O₃,

(b) from 1 to 40 wt % of CuO,

(c) from 1 to 50 wt % of ZnO,

(d) from 1 to 40 wt % of Ag,

(e) from 0 to 2 wt % of Pt,

(f) from 0 to 20 wt % of oxides of rare earth metals, elements of the3rd subgroup of the Periodic Table of the Elements or mixtures thereof,

based on the total weight of the components (a) to (e), which adds up to100 wt %,

wherein, in each case, up to half the weight of the component (a) may bereplaced by Fe₂ O₃, Cr₂ O₃, Ga₂ O₃ or mixtures thereof, of the component(b) by CoO, of the component (c) by MgO, of the component (d) by Au andof the component (e) by Pd, Ru, Os, Ir, Rh, Re or mixtures thereof,

for reducing NO_(x).

At the same time it was found, according to the invention, thatcatalysts as described in DE-A1-42 24 881 can be employed for reducingNO_(x).

In this context, the catalysts used according to the inventionpreferably comprise from 30 to 80, particularly preferably from 40 to75, in particular from 45 to 65 wt % of Al₂ O₃, preferably from 3 to 35,particularly preferably from 5 to 30, in particular from 8 to 25 wt % ofCuO, preferably from 2 to 40, particularly preferably from 5 to 30, inparticular from 10 to 26 wt % of ZnO, preferably from 2 to 35,particularly preferably from 3 to 30, in particular from 5 to 25 wt % ofAg and preferably from 0 to 1, particularly preferably from 0 to 0.5, inparticular from 0 to 0.1 wt % of Pt. As described above, the specifiedweights are based on the total weight of the components (a) to (e),which adds up to 100 wt %.

Preferably, in each case, at most 1/3 particularly preferably at most1/5, in particular 1/10 of the weight of the components (a),(b),(c),(d)and/or (e) are replaced as described above. Preferably, none of thecomponents (a),(b),(c),(d) and (e) is replaced.

Eligible for component (f) are oxides of the rare earth metals and theelements of the 3rd subgroup of the Periodic Table of the Elements,preferably oxides of 3-valent rare earth metals, in particular La and/orCe. Preferably, Pr is not present in the catalyst.

Component (f) is preferably employed in amounts of from 0 to 15,particularly preferably from 0 to 10, in particular from 0 to 5 wt %,based on the total weight of the components (a) to (e). Preferably, nocomponent (f) is present.

Particularly preferably, the catalyst consists of the components (a) to(e), specifically only of Al₂ O₃, CuO, ZnO, Ag and possibly Pt.

According to the invention a copper/zinc/aluminum oxide compound is usedwhich can be generally represented as follows: Cu_(a) Zn_(b) Al₂O_(3+a+b), where a>0, b>0, a+b≦1.

Preferably, the components (a), (b) and (c) form a spinel. Spinels aredescribed, for example, in C. W. Correns, Einfuhrung in die Mineralogie[Introduction to mineralogy], Springer Verlag 1949, pp. 77-80, H. Remy,Lehrbuch der Anorganischen Chemie [Textbook of inorganic chemistry],Akademische Verlagsgesellschaft Geest & Portig K.-G. Leipzig 1950, pp.308-311, Romp, Chemielexikon, 9th edition 1995, p. 4245. Spinelsformally derive from MgAl₂ O₄, where magnesium may be replaced by otherdivalent ions such as zinc, copper, iron. Aluminum may be replaced byother trivalent ions such as iron or chromium. In the spinel lattice theoxygen atoms form a cubic close-packed structure corresponding to aface-centered lattice. Half of the octahedral vacancies therein areoccupied by aluminum, the other half of the vacancies are empty. Oneeighth of the tetrahedral vacancies are occupied by magnesium.

What is preferably present is essentially a copper/zinc spinel. Fora+b=1 no vacant sites exist in the spinel lattice. Al₂ O₃ can act as amatrix, in which the other metal oxides are present. This is the case,in particular, for a+b≦1.

The novel catalysts may contain small amounts of SiO₂, TiO₂, ZrO₂, talcand/or cements, as long as these do not significantly affect theproperties of the catalysts. According to one embodiment the catalyst isphosphorus-free, particularly if Pr is present.

The novel catalysts have a pore volume of from 0.01 to 1, preferablyfrom 0.01 to 0.8, particularly preferably from 0.1 to 0.7 ml/g, the poresize distribution being monomodal, bimodal or polymodal. Bimodal orpolymodal catalysts in this context preferably have mesopores andmacropores. Mesopores have a diameter of less than 50 nm, macroporeshave a diameter of from 50 to 10000 nm. The catalyst preferably has abimodal or polymodal pore size distribution, from 40 to 99%, preferablyfrom 50 to 98%, particularly preferably from 55 to 95% of the porevolume being present in mesopores and from 1 to 60%, preferably from 2to 50%, particularly preferably from 5 to 45% of the pore volume beingpresent in macropores. Particularly preferably the catalyst is bimodal.

Oligomodal catalysts may also contain pores having a diameter of morethan 10000 nm, the fraction of these pores preferably being from 0.1 to20, particularly preferably from 1 to 15% of the pore volume, where theabove-specified values for the mesopores and macropores relate to theresidual pore volume.

In the case of catalysts having a bimodal or polymodal pore sizedistribution, the major fraction of the pore volume is preferably withina pore size range of from 10 to 1000 nm.

The size of the silver particles present in the catalyst is preferablyfrom 0.1 to 200, particularly preferably from 5 to 50 nm. Above 300° C.in this context, silver is present as a metal and below this temperaturemay also be present as an oxide. The weights specified above are basedon the metal.

The pore volume and the pore volume distribution are preferablydetermined by Hg porosimetry. The size of the silver particles isdetermined, for example, by means of measuring the line width in X-raydiffraction.

The BET surface area is preferably from 1 to 200, particularlypreferably from 20 to 150, in particular from 50 to 100 m² /g.

The catalysts employed according to the invention may be present in anyform, for example as pellets, tablets, which may be hollow or solid;granules having a diameter of preferably from 0.5 to 3 mm, chips,honeycombs, etc. The novel catalysts may also be present on othersupport materials such as glass fiber mats, ceramic or metallicsupports, and the supports may have various shapes, for examplecorrugated or rolled. The above-specified quantities in this contextrelate to the catalyst proper, without the additional support. Catalystsused according to the invention which are to be employed in theautomotive sector preferably are honeycomb-shaped, the hole diameterpreferably being from 0.1 to 10 mm, in particular from 0.5 to 5 mm, andthe web width preferably being from 0.1 to 5 mm, in particular from 0.3to 3 mm.

The catalysts used according to the invention can be prepared by anysuitable process. Suitable processes are described, for example, inDE-A1-42 24 881. For example, AlOOH (boehmite), CuO, ZnO and any furthermetal oxides required may be kneaded with water in the presence of abinder, extruded to form extrudates, dried and calcined. The catalystbase bodies thus prepared can be impregnated with an aqueous solution ofsilver nitrate. The impregnated catalysts are then dried and calcined.

Instead of metal oxides, the corresponding hydroxides, oxyhydrates,carbonates, salts of organic acids, nitrates, chlorides, sulfates orphosphates can be used. To prepare bimodal or polymodal catalysts it ispossible to use, instead of AlOOH, a mixture of AlOOH and Al₂ O₃,preferably γ- or δ-Al₂ O₃, with the option of employing aluminum oxide(Al₂ O₃) of different pore size distributions.

Drying preferably takes place at from 10 to 200° C., particularlypreferably from 20 to 150° C., in particular from 30 to 120° C.Calcination takes place at less than 1100° C., preferably at from 600 to900° C. Calcination after impregnation with silver nitrate solutionpreferably takes place at from 200 to 800° C.

The catalysts used according to the invention are preferably employedfor reducing NO_(x) in combustion off-gases. The reduction of the NO iseffected by reaction with a reducing agent.

The invention also relates to a process for the catalytic reduction ofNO_(x) in mixtures containing NO_(x), O₂ and hydrocarbon compounds, thereduction taking place in the presence of a catalyst as defined above,and the hydrocarbon compounds serving as reducing agents.

The mixture is preferably a combustion off-gas, the combustion off-gasin particular coming from internal-combustion motors orinternal-combustion engines.

Such an off-gas inter alia contains nitrogen oxides (NO_(x)), oxygen(O₂), water vapor and possibly hydrocarbon compounds. Hydrocarboncompounds are, for example, oxygen-containing hydrocarbon compounds suchas alcohols, ethers, aldehydes, ketones, epoxides etc. The term"hydrocarbon compounds" also subsumes hydrocarbons such as alkanes,alkenes, alkynes or aromatic compounds. Instead of the hydrocarboncompounds it is also possible to use CO or H₂. Preference is given tothe use of added hydrocarbon compounds. For example, short-chainhydrocarbons such as propene may be metered into the off-gas stream.Another preferred option is for a portion of the fuel, for example inthe case of a motor vehicle, to be supplied to the off-gas stream, sothat hydrocarbon compounds are present in the off-gas. An example of areaction taking place if propene is used as the hydrocarbon compound isshown in the following reaction equation.

    4 NO.sub.2 +NO+CH.sub.3 --CH═CH.sub.2 →5/2 N.sub.2 +3 CO.sub.2 +3 H.sub.2 O

Diesel exhaust gases, in particular, additionally contain oxygen, sincethe combustion is carried out with excess air. This means that furtherreactions may take place in which organic oxygen-containing compoundsare formed.

Hydrocarbon compounds present can therefore, on the one hand, react withoxygen present and, on the other hand, with nitrogen oxides present, thecatalyst used according to the invention preferentially catalyzing thereaction of hydrocarbons with NO_(x), compared with the reaction ofhydrocarbons with oxygen.

Exhaust gases, in particular of diesel engines, in addition to NO_(x)and hydrocarbons also contain CO, possibly soot, SO₂, and water vapor,oxygen, nitrogen (N₂) and CO₂. A diesel exhaust gas can have thefollowing composition:

NO_(x) from 10 to 10000, on average 2000 ppm

hydrocarbons from 10 to 2000, on average 200 ppm

CO from 10 to 4000, on average 100 ppm

soot from 0 to 1, on average 0.3 g/l

sulfur dioxide from 0 to 200, on average 40 ppm

water vapor from 1.5 to 8, on average 7 vol %

oxygen from 3 to 18, on average 4 vol %

CO₂ from 2 to 15, on average 3 vol %

Typical catalyst loadings are from 20,000 to 30,000, peak loadings up to100,000 m³ (s.t.p.) of gas per m³ of catalyst per hour. The invention isexplained below in more detail with reference to examples.

General preparation procedure for the catalyst

The catalyst used according to the invention can be prepared in a mannersimilar to the procedure described in DE-A1-42 24 881.

This, for example, involves 400 g of AlOOH (Boehmite, Pural® SB fromCondea), 50 g of CuO, 154.5 g of ZnO and 25 g of methylcellulose(Walocel® from Wolff, Walsrode) being kneaded with 270 g of water forone hour, being extruded to produce solid extrudates having a diameterof 3 mm and a length of 8 mm, dried and calcined for 4 hours at 800° C.The material obtained after calcination has a surface area of 54 m² /g.The porosity is 0.32 ml/g.

The inorganic constituents have the following composition: Cu: 0.63 mol(CuO: 10 wt %), Zn: 1.90 mol (ZnO: 28 wt %), Al: 6.67 mol (Al₂ O₃ : 62wt %). This corresponds to the empirical formula Cu₀.2 Zn₀.6 Al₂ O₃.8.

108 g of this material (spinel) are impregnated with 51 ml of an aqueoussolution containing 30.2 g of silver nitrate and are left for one hour.The impregnated material is dried for one hour at 120° C. to constantweight and then calcined at 600° C. The catalyst pellets thus obtainedcontain 19.2 g of metallic silver, corresponding to 17.8 wt %.

In the following examples percentages relate to weight unless otherwisestated.

EXAMPLE 1

Monomodal catalyst having the composition 20% of CuO, 20% of ZnO, 45% ofAl₂ O₃, 15% of Ag

400 g of AlOOH (Boehmite, Pural SB from Condea), 151 g of CuO and 151 gof ZnO as well as 30 g of methylcellulose (Walocel from Wolff, Walsrode)were kneaded with 320 g of water for one hour, extruded to produce solidextrudates having a diameter of 3 mm and a length of 8 mm, dried andcalcined for 4 hours at 800° C.

640 g of this spinel body were impregnated with 178 g of silver nitratein the form of an aqueous 50% strength AgNO₃ solution. This was followedby drying for one hour at 120° C. and calcination for 4 hours at 600° C.

The pellets thus obtained contained 15% of Ag, 20% of CuO, 20% of ZnOand 45% of Al₂ O₃. They were monomodal and contained pores, 95% of whichhad a diameter of 50-50000 nm. The total porosity was 0.32 ml/g.

EXAMPLE 2

Bimodal catalyst with 5% of Ag, 15% of CuO, 19% of ZnO, 61% of Al₂ O₃

The bimodal catalyst was obtained by a mixture of AlOOH and Al₂ O₃ beingused instead of just AlOOH.

611 g of Cu(NO₃)₂ ×3 H₂ O, 581 g of Puralox® SCF-A 230 (Al₂ O₃, producedby Condea) and 322 g of Pural SB (AlOOH, produced by Condea) were mixedwell for 3 hours. The dry composition was then admixed with enough waterto produce a plastic kneading composition. 45 g of formic acid were thenintroduced. The composition was kneaded for 70 minutes, formed intoextrudates, dried for 16 hours at 120° C. and calcined for 4 hours at800° C.

800 g of the body thus obtained were impregnated with 373 g of Zn(NO₃)₂×6 H₂ O, which had been dissolved in water and was made up to 400 l oftotal solution.

After 1.5 hours' impregnation, drying took place for 16 hours at 120°C., followed by calcination for 4 hours at 600° C.

As described in Example 1, the catalyst was impregnated with a 50%strength AgNO₃ solution, so that the finished catalyst contained 5% ofsilver. The catalyst had the composition 5% of Ag, 19% of ZnO, 15% ofCuO, 61% of Al₂ O₃. The BET surface area was about 100 m² /g. The wateruptake was about 0.5 ml/g, which corresponded to an overall porosity ofthe same magnitude.

EXAMPLE 3

The catalyst was prepared in a manner similar to that of Example 2, buthad the following composition: 15% of Ag, 17% of ZnO, 13.6% of CuO,54.4% of Al₂ O₃.

EXAMPLE 4

The procedure from Example 2 was repeated, but the catalyst obtained hadthe following composition: 25% of Ag, 15% of ZnO, 12% of CuO, 46% of Al₂O₃.

EXAMPLE 5

The procedure from Example 2 was repeated, but the following compositionwas obtained: 15% of Ag, 17% of ZnO, 13.6% of CuO, 54.3% of Al₂ O₃, 0.1%of Pt. Doping with Pt was effected by impregnation with an aqueousplatinum nitrate solution. This impregnation took place at the same timeas the impregnation with silver nitrate, but can also be carried outindependently.

EXAMPLE 6

The catalyst was prepared in a manner similar to the procedure describedin Example 2, but had the following composition: 15% of Ag, 25.5% ofZnO, 8.5% of CuO, 51% of Al₂ O₃.

COMPARATIVE EXAMPLE 1

For comparative purposes a catalyst was prepared in a manner similar tothat of Example 2, except that no silver was employed. The catalyst hadthe following composition: 15% of ZnO, 21.3% of CuO, 63.7% of Al₂ O₃.

COMPARATIVE EXAMPLE 2

The comparative catalyst was prepared in a manner similar to that ofExample 2, except that instead of silver a very small amount ofpalladium was employed. The catalyst had the following composition:

20% of ZnO, 16% of CuO, 64% of Al₂ O₃, 0.1 % of Pd.

Study of the catalysts

The catalysts obtained were studied as follow:

Of the respective catalysts, 10 g of chippings of the fraction from 1.6to 20 mm were introduced into a vertically positioned quartz reactor(diameter 20 mm, height about 500 mm) in the center of which a gas-permeable frit was arranged for accommodating the catalyst. The bedheight was about 15 mm. Around the quartz reactor a furnace was arrangedwhich heated the central section of the reactor over a length of 100 mm,temperatures up to 550° C. being achievable.

A gas mixture was passed through the catalyst at a flow rate of about wt10000 (1 [s.t.p.] of gas)/(1 of cat x h). The gas mixture consisted of1000 ppm of NO, 1000 ppm of propene, 10 vol % of oxygen and argon(remainder) as the carrier gas.

Downstream of the reactor the NO concentration was measured with a gasdetector, any NO₂ formed upstream of the detection being reduced in aconverter to NO. The reaction was carried out at temperatures in therange of from 200° C. to 400° C. The study results obtained are listedin the following table, including the NO_(x) concentration measureddownstream of the reactor and the minimum NO_(x) concentration and thetemperature at which the minimum NO_(x) concentration had been measured.In addition the maximum conversion ratio is listed, ie. the ratio(NO_(x) upstream) minus (NO_(x) downstream) to (NO_(x) upstream).

                                      TABLE                                       __________________________________________________________________________                                       NO.sub.x min.                              Catalyst from                                                                        NO.sub.x upstream of                                                                  NO.sub.x downstream of reactor (ppm)                                                              downstream of                                                                        T (° C.)                                                                    Conversion                     Example No.                                                                          reactor (ppm)                                                                         200° C.                                                                    250° C.                                                                    300° C.                                                                    350° C.                                                                    400° C.                                                                    reactor (ppm)                                                                        NO.sub.x min.                                                                      ratio (%)                      __________________________________________________________________________                                                   max.                           1      1000    620 400 305 390 705 300    295  70.0                           2      1000    810 300 280 600 840 235    280  76.5                           3      1000    945 480 195 365 540 190    295  81.0                           4      1000    980 520 345 820 910 345    300  65.5                           5      1000    885 575 495 725 860 485    285  51.5                           6      1000    610 385 220 415 595 215    290  78.5                           Comparative                                                                          1000    800 605 580 615 685 575    280  42.5                           Example 1                                                                     Comparative                                                                          1000    935 660 635 815 895 605    285  39.5                           Example 2                                                                     __________________________________________________________________________

The results of the table demonstrate that the catalysts used accordingto the invention according to Examples 1-6 lead to considerably betterabatement of NO_(x) compared with the comparative catalysts according toComparative Examples 1 and 2.

We claim:
 1. A process for the catalytic reduction of NO_(x), thereduction taking place in the presence of a catalyst which comprises(a)from 20 to 97 wt % of Al₂ O₃, (b) from 1 to 40 wt % of CuO, (c) from 1to 50 wt % of ZnO, (d) from 1 to 40 wt % of Ag, (e) from 0 to 2 wt % ofPt, (f) from 0 to 20 wt % of oxides of rare earth metals, elements ofthe 3rd subgroup of the Periodic Table of the Elements or mixturesthereof, based on the total weight of the components (a) to (e), whichadds up to 100 wt %, wherein, in each case, up to half the weight of thecomponent (a) may be replaced by Fe₂ O₃, Cr₂ O₃, Ga₂ O₃ or mixturesthereof, of the component (b) by CoO, of the component (c) by MgO, ofthe component (d) by Au and of the component (e) by Pd, Ru, Os, Ir, Rh,Re or mixtures thereof, wherein the components (a), (b) and (c) form aspinel which is doped with the components (d), (e) and (f), wherein thecatalyst has a bimodal or polymodal pore size distribution, 40-99% ofthe pore volume being present in mesopores and 1-60% of the pore volumebeing present in macropores.
 2. The process as claimed in claim 1,wherein the pore volume of the catalyst is from 0.01 to 1 ml/g.
 3. Theprocess as claimed in claim 1, for reducing NO_(x) in combustionoff-gasses.
 4. A process for the catalytic reduction of NO_(x) inmixtures containing NO_(x), O₂ and hydrocarbon compounds, the reductiontaking place in the presence of a catalyst as defined in claim
 1. 5. Theprocess as claimed in claim 4, wherein the mixture is a combustionoff-gas.
 6. The process as claimed in claim 5, wherein the combustionoff-gases come from internal-combustion motors or internal-combustionengines.
 7. The process as claimed in claim 4, wherein hydrocarboncompounds, CO or H₂ serve as reducing agent(s).